The present invention relates to a monitoring method for a filter mounted upstream of an electric circuit having at least one filter component with a filter characteristic, and to a corresponding monitoring device.
European Patent Application No. 0 267 548 describes a rail vehicle which is operated on a DC voltage system and which has an input filter on the system side. The input filter comprises an inductance and a capacitance. A failure of one of these two components, or even both components, has adverse effects on the response to disturbing currents.
Disturbing currents are often subject to safety limit values. Consequently, it is absolutely necessary for this filter to function dependably.
The object of the present invention is to provide a simple, reliable and inexpensive monitoring method and a corresponding monitoring device with which a continuous monitoring of the filter is possible.
The object of the present invention is achieved for the monitoring method by recording a time characteristic of at least one filter current flowing in the filter, as well as a time characteristic of at least one filter voltage present in the filter during the operation of the circuit and by determining at least one filter characteristic from the time characteristics.
The object for the monitoring device is achieved by it having at least one ammeter for recording a time characteristic of at least one filter current flowing in the filter, at least one voltmeter for recording a time characteristic of at least one filter voltage present in the filter and an evaluation unit connected to the at least one ammeter and the at least one voltmeter for determining the at least one filter characteristic from the time characteristics.
If the time characteristics of the filter current and/or of the filter voltage are averaged before the determination of the at least one filter characteristic, the influences of disturbances on the filter characteristic determined are smaller.
The filter component may be an inductor with an inductance or a capacitor with a capacitance. If the filter component is an inductor with an inductance, the inductance is determined as the quotient of a filter voltage and a change in the corresponding filter current.
In this case, the determination of the inductance can be suppressed if the absolute value of the change in the filter current falls below a limit value or the change in the filter current and/or the absolute value of the filter voltage falls below a limit value for the filter voltage. This is because the inductance is then affected by a great measuring error, so that a reliable indication of the actual inductance cannot be necessarily obtained.
If, on the other hand, the filter component is a capacitor with a capacitance, the capacitance can be determined as the quotient of a filter current and a change in the filter voltage corresponding to this. In this case, the determination of the capacitance can be suppressed if the absolute value of the change in the filter voltage goes below a limit value for the change in the filter voltage and/or the absolute value of the filter current falls below a limit value for the filter current. This is because the capacitance may then be affected by a great measuring error, so that no reliable indication of the actual capacitance can be obtained.
The filter voltage and/or the filter current or their changes may possibly be corrected by ohmic components.
The monitoring method can preferably be used in the case of an input filter of a power supply device of a rail vehicle. Consequently, the circuit may preferably be a power supply device of a rail vehicle.